238 Mr. A. Hancock on the Boring of the Mollusca into Rocks j 



Burrows frequently occur with a depression at the bottom exactly 

 agreeing in form and size with this part of the animal, and it 

 very commonly happens that there is an elevated point in the 

 depressed part corresponding with the greatest accuracy to the 

 position of the byssus. Indeed, I have seen the byssus on several 

 occasions adhering to the projection. It seems impossible to 

 have a more convincing proof than this, that the "anterior portion 

 of the animal is the excavating instrument. 



The animals of all the species that confine their operations to 

 calcareous bodies are most probably formed like those of Saxi- 

 cava and Gastrochana. Petricola is stated by Mr. G. B. Sow- 

 erby to have " the borders of the mantle thickened in front with 

 a small hole for the foot.^^ And Professor Owen, in his paper 

 on Clavagelltty published in the first volume of the ^ Zoological 

 Transactions,^ describes that genus as having the mantle closed 

 and thickened in front with a small orifice for the foot. It is 

 satisfactory to observe that this distinguished physiologist con- 

 siders that the animal enlarges its habitation by this thickened 

 portion of the mantle. I have not been able, anywhere, to find 

 a sufficient description of the animal of Lithodomus ; but from 

 the remains of it in a small specimen which I found buried in an 

 old shell, there can be little doubt that it also has the mantle 

 closed in front. 



From what has already been said respecting the holes of Pa- 

 telltty it is sufficiently evident that they also partake of the form 

 of the animal. 



The direction of the burrows of the Acephala, as well as their 

 form, corroborates the opinion that the anterior part of the ani- 

 mal is the boring apparatus. I have examined a great number 

 of the burrows of these animals, and find that the direction of the 

 excavations is always inclined a little to one side : none of them 

 are at right angles to the surface by which the animal enters. 

 The bores of Teredo are so inclined at the commencement ; but 

 their course is soon altered, and is afterwards, apparently, deter- 

 mined at the will of the animal. The burrows of Pholas and 

 Saooicava are generally continued their entire length in the ori- 

 ginal direction ; they are, however, sometimes slightly twisted as 

 well as inclined. This happens with Saxicava more frequently 

 than with Pholas, and is occasioned by the animal having turned 

 a little more to one side than usual. This obliquity of the bur- 

 row is unfavourable to the opinion of rotatory motion, and arises 

 from the exposure of the animal towards the ventral margin of 

 the shell. The cutting surface being therefore placed to one side, 

 the excavations cannot be perpendicular unless complete rotation 

 were to take place, which we shall afterwards see is unnecessary* 



It now remains to be shown that the anterior parts of the ani- 



